5. Let S = {1,2,3, 4}. The following are four relations from So S. For each relation, determine if it is a function from S to S. Give a short explanation for each question. (a) {(1, 2), (3, 4), (4, 1)}. (b) {(1,2), (2,3), (3, 4), (4, 1)} (c) {(1, 1), (1,2), (2, 1), (2, 2), (3, 3), (3, 4), (4, 3), (4, 4)} (d) {(1,2), (2, 2), (3,2), (4, 2)}